Off-road vehicles now pose the single biggest threat to America's public lands and represent a fast-growing law enforcement problem.

That's the verdict of a new coalition of former public land managers and rangers, which has formed to bring attention to the problem.

Understaffing, weak penalties, and lack of enforcement of trail restrictions, among other problems, have led to environmental degradation and an increasingly chaotic environment at many popular federal recreation areas that are overwhelmed by the sheer numbers of off-road-vehicles (ORVs), the coalition says.

"These things are just crawling all over the place, unregulated, damaging the environment and wreaking havoc - there's no teeth in any law enforcement," says Jim Baca, a member of the Rangers for Responsible Recreation coalition and a former director of the Bureau of Land Management (BLM) under the Clinton Administration. "Congress needs to look at this and make sure public land agencies are doing their job."

BLM officials acknowledge the rising numbers of ORVs, but insist the problem is under control.

Use of ORV is legal on designated roads and trails across more than 80 million acres of land overseen by the BLM and millions more overseen by the US Forest Service. But ORV use, especially in the western US, has zoomed off trails into fragile areas, growing far faster than public land managers' ability to police it, the coalition says.

National growth numbers are hard to come by. But the most popular areas risk being overrun, a BLM spokesman in Washington acknowledges. One popular BLM-run recreation area, Imperial Sand Dunes in California, has ORV visits soar to 1.2 million each year, BLM officials say. Both the BLM and US Forest Service are taking steps they say will reign in ORV infractions.

Indeed, conflicts, sometimes violent, appear to be growing between rangers and ORV users who flout trail signs and damage delicate desert and other habitat, or lug alcohol and drugs into wilderness areas, the group says. But a BLM spokesman says law-enforcement incidents involving "off-highway vehicles" (OHVs) actually fell from 5,846 in 2004 to 5,066 last year.

"We realize there has been exponential growth in OHV use, so we know it's a challenge. But we think we're meeting the challenge," says Tom Gorey, a BLM spokesman.

Yet the problem is clearly growing more serious in many locations. At an Easter weekend gathering, 50 federal rangers faced "near riot conditions" with about 1,000 out-of-control ORV enthusiasts at the Little Sahara Recreation Area in Utah. Revelers sexually harassed a number of women among the 35,000 people using the area, federal reports of the incident show. There were 300 arrests and 37 injuries.

"What has been lacking is the assurance of tough enforcement and the backbone needed to bring the runaway problem under control," said Jim Furnish, former deputy chief of the US Forest Service and a member of the rangers' coalition, in a statement. "Folks visiting our public lands expect enforcement that protects natural resources, ensures visitor safety, and reclaims a family friendly atmosphere."

As numbers grow, the environmental challenge is growing, too.

At the Tellico Off-Road Vehicle Area within the Nantahala National Forest in North Carolina, ORV use has been allowed to soar beyond planned limits, critics say. Now, deeply-rutted trails wash silt into once-pristine trout streams that feed into the Tellico River.

In a "notice of intent" letter last week, the Southern Environmental Law Center informed the US Forest Service it would soon file suit on behalf of several trout fishing organizations to enforce the federal Clean Water Act. The letter says that trails close to streams violate the law.

Enthusiasts of ORV riding say the problem nationally is due to a lack of adequate mapping and signage on public lands to show users where to ride, and that new forest service and BLM rules are addressing it.

But as numbers grow, broader rider education efforts are needed, they acknowledge.

"We recognize there are issues with OHV use on public lands," says Jack Welch, president of the Blue Ribbon Coalition, which represents off-road enthusiasts and vehicle manufacturers. "We believe most of these problems can be solved with better signage and map programs. But we also think the ranger group is not really interested in working together with us – they simply want to eliminate motor recreation on public lands."

Coalition members deny they are just out to eliminate motorized vehicle use, but say they want the ORV problem brought under control.

Penalties that are severe enough to deter violators are badly needed, such as forfeiture of hunting and fishing licenses or confiscation of vehicles when off-roaders are caught ignoring the law, they say. A larger budget for more rangers to enforce the law is also required, they add.

"Land management agencies just don't have a lot of enforcement capability right now," Mr. Baca says.